This invention relates to pigmented coating compositions, such as paints, inks and the like and, more particularly, to coating compositions containing nonleafing aluminum flake pigments.
A coating composition as herein contemplated is a composition that can be applied to a substrate to establish an adherent film thereon, e.g. a protective or decorative finish coating that covers an extended surface area, or a small imprint or marking, e.g. ink. Most coatings comprise a polymeric binder, a pigment if appropriate, and (in the case of liquid coating compositions) a thinner or solvent. Pigmented coating compositions contain a dispersion of a particulate solid pigment in a binder either with or without additional ingredients. The term "vehicle" is used herein to designate material which can be either solid or initially liquid in form, that is inert with respect to the pigment, and (when applied to a substrate surface) forms a substantially continuous surface-adherent coating film that binds and/or holds the dispersed pigment particles. The most commonly used coating compositions are initially in substantially "liquid" form.
Typically, liquid vehicles comprise a mixture of a viscosity-reducing solvent or thinner (either polar, e.g. water, or non-polar, e.g. hydrocarbon) and a film-forming (e.g. polymeric) binder, both of which may themselves be constituted of plural ingredients. As the pigmented liquid composition is applied to a substrate, the solvent or thinner evaporates and/or is absorbed by the substrate and/or polymerizes to form a more or less transparent coating film. The pigment, which remains dispersed in the film, serves to impart properties to the coating composition such as opacity, color, brightness and the like.
The solvent used in application of a particular coating composition will depend primarily on the binder system and the application. For example, hydrocarbon-based solvents find wide application with organic (lipophilic) resin/binders; while water (hydrophilic) based and/or emulsion-forming resins such as latex are used with aqueous solvent systems.
On the other hand, "dry" or powder coatings do not use a liquid vehicle for carrying the binder and/or pigment to the substrate. Powder coatings are of many types having, for example, thermosetting and/or thermoplastic binders including a wide variety of polymeric materials. Epoxy, polyester and like resins can also be applied in dry or powder form, with or without pigment. The powder coatings containing metallic pigment, and especially aluminum powder pigment, are more fully described in Edwards and Wray, Aluminum Paint and Powder, 3rd. ed., Reichhold Publishing Co., New York, N.Y. 1955, pp 5-11.
The methods of applying powder or dry pigment vary with the substrate and end use. Spray dispersion with subsequent thermal treatment is most common, but (depending on conductivity of the substrate) electrostatic and fluidized bed methods are also currently in use. Examples of such coatings and their method of application are shown in U.S. Pat. Nos. 4,003,872, 4,205,665 and 3,980,607.
Coating compositions containing metal flake pigments are used to impart a metallic luster to the coated article, whether initially liquid or otherwise. These pigmented compositions find their greatest demand in finish coating for topical applications. Coatings containing aluminum flake pigments are widely used in a myriad of applications. For example, such coatings are used in inks, plastics, paper and fabric finishes, ready-mixed paints, aerosol paints, maintenance and industrial coatings, automobile topcoats and many other applications where a metallic luster is desired. Thus it is known that aluminum flake pigments can be incorporated in a diversity of binder systems, with or without solvents, to provide protective and decorative finish coatings having metallic luster and/or silvery color.
Aluminum flake pigments used in coating materials are generally of two types; "leafing" and "non-leafing". Both "leafing" and "nonleafing" aluminum flake pigments are constituted of minute flakes of aluminum or aluminum-based alloys. The property of "leafing" may be defined as the tendency of metal pigment flakes, when dispersed in a coating composition vehicle (whether or not in the presence of a solvent) having sufficiently high surface tension (and free of substances that inhibit leafing), to become arranged in flat, parallel or overlapping relation at the surface of an applied coating layer of the composition, so as to provide a highly reflective coating that simulates the appearance of bare metal. Thus, leafing aluminum pigments are those in which the proportion of flakes that "leaf" is great enough to produce this visual effect, as is desired for a variety of applications.
Nonleafing aluminum flake pigments, on the other hand, exhibit relatively little or no leafing. These flakes, when dispersed in an applied coating layer, are at least predominantly disposed in random attitudes and at random distances from the coating surface, providing a less reflective coating than a leafing pigmented coating. As explained herein below, the leafing or nonleafing character of a pigment is determined primarily by the method of manufacture of the flake and the use of particular materials in subsequent treatment of the flakes. The extent to which a particular flake material exhibits these characteristics in a coating composition can depend upon the binder and solvent system.
Nonleafing aluminum flake pigments are employed when metallic luster without bare metallic appearance is desired in a coating. In addition, they are especially preferred for thin-coating applications (e.g. of the order of 0.5 mils) and for coatings subject to abrasion and/or weathering which might impair, remove or otherwise move the surface-adjacent array of flakes of a leafing pigment. Notwithstanding the diverse uses of these nonleafing pigment containing coatings, current commercially available compositions are attended with inferior (as regards the nonleafing pigments) or undesirable properties.
The prior art pigmented coating compositions containing nonleafing aluminum flake manifest problems ranging from difficulty in manufacturing the nonleafing flake pigments; to difficulty in dispersing the pigment in liquid vehicles (binder and solvent); to problems with the coating surface finish.
In respect to the production of leafing and nonleafing aluminum flake pigments, both involve the reduction of particulate aluminum (such as foil scrap or atomized aluminum powder) to the desired minute flake form. This is conventionally accomplished by subjecting the particulate aluminum to the action of a ball mill, stamping mill or other equipment capable of flattening and breaking up the particles, in the presence of a minor amount (based on the weight of aluminum) of a so-called milling agent. For convenience of reference, the operation of reducing particulate aluminum to flake form will herein be termed "milling," and the equipment in which it is performed will be termed a "mill," regardless of the specific nature of the operation and equipment used. Milling may be performed either dry (in air or other gas) or wet (in a liquid wet-milling vehicle such as mineral spirits). After milling, the flake particles may in some instances be subjected to various additional treatment.
Presence of the milling agent in the milling operation is essential to protect the aluminum particles during reduction so that they are flattened into flake form rather than being merely broken up, and to prevent cold welding of the particles. In addition, the milling agent covers the particle surfaces with a thin layer of material which remains on the particles after milling, protecting them from corrosion, oxidation or other deleterious attack so as to aid in preserving the brightness or luster of the produced pigment. The protective coating layer also reduces the hazard of fire or explosion incident to handling aluminum powder. The material of this layer is herein termed "milling agent residue" because it consists essentially of the milling agent compound, compounds present during milling and/or derivatives thereof produced by reaction in the course of or as a consequence of the milling operation. It is believed that some milling agent residue of the described layer may be chemically bonded at the flake surfaces. In any event, some of the layer of residue is in direct contact with elemental metal at the flake surfaces, whether or not such contact involves chemical bonding, because the milling agent is present at the time the elemental metal is exposed at these surfaces by milling.
Leafing aluminum flake pigments are made by using as the milling agent, one or more substances herein termed "leafing milling agents" which are known to impart leafing properties to the flakes. Currently used leafing milling agents enable milling to be performed with high efficiency. The desired sizing of leafing flake pigments thus produced is readily controllable, for example, by variation in resident time of the charge of aluminum in the mill. Consequently, the manipulative operations involved in making leafing aluminum flakes are advantageously simple, straightforward and convenient. These manipulative operations (apart from the specific milling agent employed) will be termed "leaf milling" herein. It will be understood that the term "leaf milling" designates that milling operation (whether in a ball mill or other equipment) which, if performed in the presence of a leafing milling agent such as stearic acid, would result in production of a leafing flake pigment.
The methods heretofore known for production of nonleafing aluminum flake pigment are not comparably facile or effective when compared to leaf milling techniques. Likewise the nonleafing pigment product (either because of or in spite of these production methods) is attendant with problems not present in utilizing leafing flaked material. In one of the production procedures, known as chemical deleafing, particulate aluminum is first milled in the presence of a leafing milling agent such as stearic and/or palmitic acid. The resultant flakes are then treated with deleafing agents such as lead napthenate or octoate, aqueous phosphates or acetic acid. Another procedure involves using a nonleafing milling agent, typically an unsaturated acid such as oleic acid, in the milling operation, instead of a leafing milling agent. Chemical deleafing is disadvantageous from the standpoint of operational convenience, because it requires an extra, deleafing step after milling. Additionally, the deleafing agents, which remain in the product, are considered undesirable contaminants or pollutants in at least some coating applications.
Use of known nonleafing milling agents, in particular oleic acid, also presents serious drawbacks, because, while these agents produce a nonleafing product in a single step, their effectiveness as milling agents is poor. Consequently, milling times must be short, as compared with leaf milling (to avoid product degradation) and production efficiency is very low. The milled metal commonly contains high levels of oversized particles, amounting sometimes to as much as 30% of the total feed and necessitating successive screenings to obtain a properly sized pigment. Remilling of the oversized particles is even less efficient. Use of oleic acid also results in lower extent of protective covering of the flakes than is provided by leafing milling agents. Consequently the hazard of explosion is increased, preventing production of dry pigments and making more difficult the attainment of very fine particle size grades.
Currently commercially available nonleafing aluminum flake pigments, as produced by the abovedescribed procedures, have substantial shortcomings tending to impair the quality of coating compositions (both in the liquid vehicle and as a finish) in which they are incorporated. Coating compositions containing pigments produced with an oleic acid milling agent tend to form insoluble agglomerates upon standing for any length of time (whether a liquid solvent-binder system or otherwise). Thus, the applied coating tends to exhibit an undesired graininess. Progressive agglomeration often renders the coating composition virtually unusable (not able of standard application) after relatively short periods of storage. Additives used to inhibit agglomeration, even when effective, are known to have adverse effects. Finish coatings containing chemically deleafed pigments (even if not agglomerated) are dull and relatively unattractive (possibly due in some respect to etching of the flake surfaces by the deleafing agent). Moreover, nonleafing pigment containing coating compositions are known to lack desirable product characteristics such as tinting strength, opacity and luster. As a result, excess pigment is required to give the coating the required hiding strength. This excess, in turn, causes dullness of the finish luster.
Alternative proposals for production of nonleafing aluminum flake pigments have been set forth in U.S. Pat. Nos. 2,858,230 (treatment of leafing pigment with an aqueous solution containing available PO.sub.4 ion), 3,264,129 (use of certain aliphatic amines as milling agents), and 3,389,105 (use of fluorocarbon resins as milling agents). These proposals, however, have not found commercial acceptance as ways to ovecome the problems associated with current nonleafing aluminum flake pigments and their manufacture.
Thus it can be seen that coating compositions containing nonleafing aluminum flake pigments, while highly desirable in finish coatings which are subject to abrasion and/or weathering are frought with problems not easily solved by a mere variance of parameters such as ingredients, method of application or the like. For example, in the manufacture of the nonleafing pigment material, short milling times give more uniform reduction of product but multiple screenings are required to provide an acceptable pigment material. Further, in order to alleviate seediness in the finish, screening through a 400 mesh (Tyler) sieve is often required to provide a finish having sufficient luster and depth. However, the finer screening accelerates agglomeration of the nonleafing pigment material in the liquid vehicle which requires expensive application methods, e.g. continual mechanical manipulation of the liquid material to be applied. Stabilizer and anti-agglomerating agents are necessary to give the liquid coating product an acceptable shelf life. These additives, however, are deleterious to certain properties of the coating finish.
Further, to overcome the "seediness" or grainy appearance of finishes containing standard nonleafing aluminum pigment, thicker coatings must be applied. This not only leads to undue expense, but also produces undesirable characteristics in the coating finish. Additional disadvantages of the prior art coatings are manifested in tinting strength (hue produced by a given volume of pigment in a coating), opacity (hiding ability of a given volume of pigment), metallic luster or brightness, and flop (change in hue or lightness with change in viewing angle).
Finally, because of the tendency to agglomerate (especially in the finer grain pigment liquid coating material), the hiding power (opacity) of prior art pigmented coatings is reduced requiring an additional amount of nonleafing pigment material. Generally, the amount of aluminum used per volume of liquid vehicle varies, but excess amounts of pigment tend to dull the gloss of the finish as well as affect the application characteristics of the coating composition. Excess pigment can result in a film disfigurement known as mottling, flocking or flooding. This is due partly to the pigment flake size and partly to the nature of certain binder materials which tend to form migrated colonies of pigment.
Thus a pigmented coating composition (and especially liquid coatings) containing nonleafing aluminum flake that is easy to apply, does not agglomerate upon standing (without the use of stabilizer, etc.) and produces a smooth, non-grainy finish when applied to a substrate would be desirable. Additionally, it would be advantageous if the method of producing the nonleafing aluminum pigment material of the coating composition were as economical and facile as that of producing leafing flake material. It would be of further advantage if the nonleafing aluminum pigment material could be supplied to the liquid vehicle as a dry powder rather than as a paste, since the mineral spirit vehicle could be deleterious to the liquid coating composition and/or the finish coating composition.
U.S. Pat. No. 3,781,177 teaches that admixture of isostearic acid with previously milled aluminum flake powder (either leafing or nonleafing, i.e. presumably already bearing a layer of milling agent residue) agglomerates and thereby dedusts the powder for explosive use.